Clear ringbuffer after dumping to file

Clear the in-memory ringbuffer after writing to file.

Bug: 193007899
Test: Manually verified ringbuffers are cleared with command "adb shell
lshal debug android.hardware.wifi@1.5::IWifi"

Change-Id: Icfa08634e948d7155e231458edd394a4d699fbaa
diff --git a/wifi/1.5/default/ringbuffer.h b/wifi/1.5/default/ringbuffer.h
index d8b87f2..03fb37a 100644
--- a/wifi/1.5/default/ringbuffer.h
+++ b/wifi/1.5/default/ringbuffer.h
@@ -37,6 +37,7 @@
     // within |maxSize_|.
     void append(const std::vector<uint8_t>& input);
     const std::list<std::vector<uint8_t>>& getData() const;
+    void clear();
 
    private:
     std::list<std::vector<uint8_t>> data_;